The price of Steem your account is using to work out the value of your holding is based on the average of the last 7 days median price (I believe). So, if the price has fallen in the last 7 days, your account will be lagging the real market, and thus appear to be falling in value.

To get a better reflection of your account value, work it out yourself (i.e. Steem Power x Current Price)..